Some sensors for use with an automatic door employing light-emitters and light-receivers survey a surveillance area with a large number of light spots by means of a small number of light-emitter and light receivers. For example, an automatic door sensor disclosed in Patent Literature 1 employs a segmented lens through which light from light-emitters is projected onto different locations in a surveillance area, and light reflected from these locations are condensed through another segmented lens onto light-receivers. According to Patent Literature 2, two light-emitters are disposed side by side, being spaced from each other by a distance larger than the distance between the centers of two lens segments of a two-segmented lens. The two light-emitters are so disposed that light from the two light-emitters can pass through the centers of the two lens segments, respectively, whereby the light is projected onto locations on opposite sides of an imaginary center line passing through the midpoint between the two light-emitters. A surveillance area is formed in front of an opening of the automatic door. In order to avoid unnecessary opening and closing of the automatic door, it is necessary for the width dimension of the surveillance area in the direction along the width of the opening to be adjusted depending on the width of the door opening and the height at which the sensor is mounted. The adjustment is made in Patent Literature 1 by disposing a light-blocking member on a portion of the segmented lens for the light-emitters through which light would otherwise be projected in an undesired direction and by disposing a light-blocking member on a portion of the segmented lens for the light-receivers through which reflected light from an undesired direction would otherwise be condensed. According to the technology of Patent Literature 2, that one of the light-emitters which can project light in one of the two sides of the imaginary center line to which it is not desired to project light is turned off to thereby turn off the surveillance area. A sensor disclosed in Patent Literature 3 includes a number of light-emitting devices, i.e. light-emitters, arranged in a matrix so that light can be projected to form a matrix in a surveillance area, and a number of light-receiving devices, i.e. light-receivers, disposed in a matrix to correspond to the respective ones of the light-emitters so that the light-receivers can receive light the respective corresponding light-emitters project onto the surveillance area. A row of light-emitters is caused to emit light, and a row of light-receivers including those corresponding to desired ones of the light-emitters in the row is caused to receive light, to thereby make it possible to detect a human body or the like at a desired location.    Patent Literature 1: JP3004867U    Patent Literature 2: JP2000-304869A    Patent Literature 3: JP2007-271537A